This invention relates to a method and apparatus for separating air.
In a modern air separation plant, a stream of air is compressed and has components of low volatility such as water vapour and carbon dioxide removed therefrom. The resultant purified air stream is cooled by heat exchange with returning streams to a cryogenic temperature suitable for its separation by rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column. Most if not all of the air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and nitrogen vapour. The nitrogen vapour is condensed in a condenser-reboiler which links the higher pressure column with the lower pressure column, the condensation being performed by heat exchange with liquid oxygen collecting in the bottom of the lower pressure column, the liquid oxygen thereby being reboiled. A part of the resulting liquid nitrogen condensate is used as reflux in the higher pressure column, and the remainder is withdrawn from the higher pressure column, is sub-cooled, and is passed through a pressure reduction of throttling valve into the top of the lower pressure column and provides the reflux for that column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column is sub-cooled and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid is separated into oxygen and nitrogen products in the lower pressure column. These products are withdrawn in the vapour state from the lower pressure column and form the returning streams against which the incoming air stream is heat exchanged.
Such an air separation process is performed at cryogenic temperatures. Notwithstanding the thermal insulation of these parts of the air separation plant operating at below ambient temperature, there is a need for refrigeration to be generated to compensate for heat "inleak" into the plant. This need is normally met by expanding a minor stream of the purified air in a turbine with the performance of external work. This work may be to drive a booster-compressor which feeds the expansion turbine with purified air for expansion at a pressure in excess of that at which the higher pressure column receives the main flow of air for separation. The turbine, may if desired, exhaust into the lower pressure rectification column.
Typically, the nitrogen and oxygen products of the air separation are required or produced at a little in excess of atmospheric pressure. Thus, the operating pressure at the top of the lower pressure column is conventionally a selected pressure in the range of 1 to 1.5 bar. This pressure in turn governs the pressure in the higher pressure column since the two columns are linked by the condenser-reboiler. There is sometimes, however, a need to provide an oxygen product at a pressure well above atmospheric pressure. In such circumstances it can be advantageous to operate the lower pressure column at a pressure above 2 bar. On the same or other occasions, air is available at a pressure typically in the range of 10 to 20 bar as a bleed from the air compressor of the gas turbine. It is convenient and advantageous on these occasions to operate the higher pressure column at substantially the pressure produced by this air compressor, and such a higher pressure column operating pressure entails a lower pressure column operating pressure well above 1.5 bar. Indeed, US-A-4 224 045 discloses that the optimum power consumption for the air separation process is when the higher pressure rectification is in the order of 10 bar and that the power consumption is favourable even at considerably higher pressures.
The favourable economics of operating an air separation plant such that the oxygen and nitrogen products are produced at a pressure in excess of 2 bar from the lower pressure column are dependent upon there being a use for both products at this production pressure or one higher. In general, there is a growing demand for large, continuous flows of high pressure oxygen in such processes as coal gasification, partial oxidation and the manufacture of iron by direct reduction. There is rarely however a complementary demand for high pressure nitrogen on the site where the oxygen is used. Nonetheless, if the source of the air for separation is a bleed from the air compressor of a gas turbine, these are typically opportunities for recovering power from the pressurised nitrogen product by expanding it in the expander of the gas turbine. Such use of nitrogen helps to increase the overall power output, to compensate for the air bled from the air compressor of the gas turbine, and, if the nitrogen is introduced into the combustion chamber of the gas turbine to reduce the formation of oxides of nitrogen in the combustion products. Such use of nitrogen, is for example, disclosed in US-A-4 224 045, US-A-4 557 735, US-A-4 806 136 and EP-A-0 384 688.
It is not always however economically feasible to operate a gas turbine on the site where the elevated pressure oxygen product is produced. In addition, it is not always technically desirable to introduce large quantities of nitrogen into the combustion chamber or expander of a gas turbine. Further, even when there is a demand for nitrogen for expansion in the expander of a gas turbine, this demand may fall substantially short of the production of nitrogen in the air separation plant. Accordingly, there is a need for ah air separation method and apparatus which is able to produce oxygen at an elevated pressure but which is at the same time able to minimise the rate of production, if any, of gaseous nitrogen product at elevated pressure. It is an aim of the present invention to provide a method and apparatus that meet this need.